Mobile offshore drilling units (MODUs), floating production platforms (FPPs), SPARS, and similar vessels are often moored at offshore venues utilizing either preset catenary mooring lines or preset taut-leg mooring lines. Mooring is accomplished by first attaching a plurality of preset mooring lines to the vessel to be moored, and thereafter applying a predetermined tension to each of the mooring lines. Heretofore the necessary tension has been applied to the mooring lines utilizing winches mounted on the vessel to be moored.
Winches having sufficient capacity to be used in tensioning mooring lines of the type used in offshore mooring operations are exceedingly expensive, but are used only intermittently, such as during the initial mooring of the vessel upon which they are installed, during relocation of the vessel, and in response to changing conditions at the mooring site. Thus, a need exists for method of and apparatus for mooring MODUs, FPPs, SPARs, and similar vessels at offshore venues which does not require the installation of mooring winches on the vessel to be moored.
Offshore mooring can also be accomplished using a technique known as dynamic positioning. The dynamic positioning technique involves the use of one or more propulsion devices mounted on the vessel to be moored which are operated to maintain the vessel at a specified location as determined by GPS technology. Although generally satisfactory in operation, dynamic positioning systems require the consumption of substantial amounts of fuel and are therefore expensive to operate. The fuel consumption necessary in the operation of dynamic positioning systems also raises environmental concerns.
The present invention comprises a method of and apparatus for offshore mooring which fulfills the foregoing and other requirements long since found lacking in the prior art. In accordance with a first embodiment of the apparatus of the invention, a clamping apparatus is provided at the distal end of either a preset catenary mooring line or a preset taut-leg mooring line. A wire or chain pendant extends from the vessel to be moored through the clamping apparatus. The distal end of the pendant is connected to a line which extends to a winch mounted on an anchor handling vessel. The winch is utilized to pull the pendant through the clamping apparatus until a predetermined tension is exerted on the mooring line and the pendant. The clamping apparatus is then actuated to securely clamp the pendant to the mooring line while maintaining the predetermined tension. In this matter the tension necessary to properly moor the vessel at the offshore site is achieved without requiring the use of mooring winches on the vessel being moored.
In accordance with more specific aspects of the invention, a remotely operated vehicle (ROV) is deployed from the anchor handling vessel. A chain is mounted at the distal end of the pendant, and a submersible buoy is mounted at the distal end of the chain. The line deployed from the winch on the anchor handling vessel is secured to the pendant during tensioning operations.
The chain portion of the pendant extends around a pulley or sheave mounted on the clamping apparatus. When the predetermined tension has been applied to the mooring line and the pendant by the winch on the anchor handling vessel, the ROV actuates the clamping apparatus to securely clamp the pendant to the mooring line while maintaining the predetermined tension. Thereafter, the ROV disengages the vessel from the pendant.
In accordance with a first embodiment of the method of the invention, a clamping apparatus is mounted at the distal end of a preset mooring line. The clamping apparatus is brought on board an anchor handling vessel. A pendant extending from a vessel to be moored is directed through the clamping apparatus and is secured to a line extending from a winch mounted on the anchor handling vessel. The clamping apparatus having the pendant extending therethrough is then lowered into the sea, and the winch on the anchor handling vessel is operated to apply increasing tension to the pendant and the mooring line until a predetermined tension is achieved. An ROV deployed from the anchor handling vessel is then utilized to actuate the clamping apparatus to securely clamp the pendant into engagement with the mooring line while maintaining the predetermined tension, and thereafter to disengage the pendant from the anchor handling vessel.